Undersized NG tankless - supplement or replace?

Hi Everyone
I'm new to the forum, but it looks like a really great resource. I'm hoping to get some feedback on several questions about our tankless heater - repair/supplement/replace. Thanks in advance!

We just bought a 2 bathroom apartment with a natural gas tankless heater. Long story short, we have found out that the unit is only spec'd to provide 2.5GPM for 60* rise, which isn't adequate for 2 simultaneous showers. I can't understand why such a small unit would have been installed here. What size/flow-rate unit is appropriate for a 2 bathroom apartment?

If I restrict the adjustable shower heads so they are totaling about 3GPM of 105* water, then we can take 2 quite weak showers, but at least at a good temp. A plumber came out and told us that our heater was installed without the recommended flow restrictor/selector of 2.5GPM on the inlet, and that's the only tweak that might help our situation a bit. If we install that, I presume it will have similar effect to my restricting the shower heads - a drop in flow, but maintaining temp. Is this right?

Is there a way to supplement the existing heater without replacing it? For example, a small electric unit after the heater and before the mixing valve, to add a few degrees if the temp from the main heater drops too low (because the flow rate is too fast for adequate heat exchange in the main unit's coil).

We have some money in escrow from the seller to repair this, but I don't think it would cover the cost of a complete replacement. Can anyone tell me what a 4-5GPM heater would cost to professionally install? From seeing heater costs at home depot, and looking at the amount of work to install, I expect heater plus installation should be $2-3k total, but I see numbers much higher than that, and I'm not sure why. Perhaps after some energy credits and selling the old heater that still has lots of life (is that possible?) we might recover some of our out of pocket cost above our escrow...

They won't do squat for tub-fills since the drain & HW have to both run at the same time to get the benefit, but they're quite good for gaining showering capacity. Better still, as a system a tankless + DWHR unit together have an "apparent efficiency" well over 100%, since it's taking back heat that was otherwise quite literally running down the drain. It uses no fuel, it's a passive device, but it definitely works, at far lower cost than adding or replacing a tankless.

Key to getting the full benefit, the DWHR unit has to feed the cold side of both the tankless AND the shower. That's often most easily achieved by having it feed the entire cold water distribution to the house. The output temp of the DWHR in summer may hit the mid-70s F when the shower(s) are running, but otherwise has no effect on the cold water side. If you have to choose either feeding the cold side of the shower or cold side of the tankless, not both, feed the tankless. That reduces the recovery efficiency of a ~50% unit to about 40%, but that's still enough to get the showering performance you're looking for.

Some tankless systems are designed where they can be installed in series and talk to each other. Essentially, add a second one, the first acting like a preheater when the second can't keep up. A small electric tank would quickly run out. Replace the existing tankless with a higher BTU input version. These things have a relatively fixed temperature rise. In the winter, when the incoming water can be quite cold, that rise just isn't enough. Sounds like you have a candle when you need a blowtorch.

I was thinking of a small electric tankless. For example, I saw one for $200 that would give a 20 degree rise at 3GPM, which would probably bridge my gap. It would only kick in occasional (when the NG tankless output got low). The problem is that model (and probably most) requires 240V wiring which I don't have in that closet, and opening walls to run wires would be a huge project in itself.

The electric tankelss at 3gpm is ~1500 lbs/hr, times 20F is 30,000 BTU/hr (or about 9000 watts>

That's 105 KBTU/hr,total, not a whit more, which is plenty for a single shower flow, but WAY under sized even for two summertime showers. In winter your incoming water temps are probably hitting around 40F, maybe a bit under, and showering temps are in the 105F range, for a 65F delta-T. With two water-sipper heads at a total of 3gpm that's 1500lbs/hr x 65F= 98,500. If one of the showerer's prefers a 110F (pretty damned hot), or the incoming water temps hit 35F during a cold snap, one or both of them is going to be just as unhappy as before.

Bottom line, you'd need to roughly double the output of your gas heater to have any margin (which is why a ~50% heat exchanger would be ideal), and a 9kw tankless just doen't have the brawn to get you there.

There is one vendor (EcoDrain) of a flat-plate type drainwater heat exchanger that might get you there if you don't have sufficient vertical drain for a gravity-film type. It's not listed on the third party tested NRCan rebate eligibility site, but there's no reason to think they don't actually work. With plate-type heat exchangers there may be clogging issues due to the plate spacing issues (they claim to have that solved but...), and one per shower would almost certainly have sufficient thermal-oomph to clear the hurdle.

Gravity film type drainwater heat recovery heat exchangers in the 50% range will deliver about a 25-35F higher temperature rise, at any flow rate (higher at very low flow rates), and they don't use any power. Too bad you don't have the space.

Ensure your gas supply is adequate for 200K BTU (pipe size and distance, other appliances on the line) and install a 200K BTU unit. Don't do some bizarre patch job like adding a second small unit. Fix it right and be done with it. Given you already have a tankless install replacing it should not involve huge labor, assuming the gas piping is adequate.